1. Field of the Invention
The present invention relates to a data conversion table that outputs values of at least one piece of converted data according to each of a plurality of combinations of values of three different pieces of color component data. More specifically the present invention pertains to a method of changing a data conversion table, in order to change values of converted data for other combinations with a change in value of converted data for a specific combination among the plurality of combinations of the values of three different pieces of color component data.
2. Description of the Related Art
In the field of printing and prepress, the user generally desires to check the results of printing before producing a mass of prints with a printing machine. Prior to mass printing, the typical procedure produces a small quantity of prints using an economy proof press, such as a thermal sublimation printer or an ink jet printer, to check the results of simulated printing. The color range expressed by prints produced with a printing machine is, however, different from the color range expressed by prints produced with an economy proof press. When image data to be printed are directly given to the economy proof press, it is thereby difficult to check the results of simulated printing accurately. The general procedure accordingly carries out color conversion of image data before feeding the image data to the economy proof press. In accordance with a concrete process, color component data consisting of the image data are input into a look-up table (hereinafter referred to as LUT) for color conversion, and fed to the economy proof press after the color conversion. This makes the color range expressed by the prints produced with the economy proof press substantially identical with the color range expressed by the prints produced with a printing machine. Such color machining enables the economy proof press to give the similar results of printing to those of the printing machine. In the description below, the device such as a printing machine is referred to as a target device, and the device such as an economy proof press as a simulation device.
FIG. 24 is a block diagram illustrating a color conversion apparatus with the LUT for color conversion discussed above. The color conversion apparatus of FIG. 24 primarily includes an LUT element with the LUT for color conversion and an LUT generating element for creating the contents of the LUT for color conversion. The LUT element mainly includes an input one-dimensional LUT 20 for canceling .gamma. correction made on every color component data, cyan (C), magenta (M), yellow (Y), and black (K), a three-dimensional LUT 22 functioning as the LUT for color conversion, and an output one-dimensional LUT 24 for making .gamma. correction on every output color component data C, M, Y, and K. The LUT generating element mainly includes a processing circuit 42 for formation of a polynomial (1+C+M+Y)n of three primary color component data, cyan (C), magenta (M), and yellow (Y), a matrix arithmetic operation circuit 26 for transforming the three primary color component data C, M, and Y to L*a*b* data in an CIE LAB color space, and a gamut mapping circuit 28 for, in case that a specific color can not be reproduced by the simulation device, determining a substitutive color that is visually closest to the specific color and can be reproduced by the simulation device. The LUT generating element further includes a processing circuit 44 for formation of a polynomial (1+L*+a*+b*)n of L*a*b* data, a matrix arithmetic operation circuit 30 for transforming the L*a*b* data to three primary color component data C, M, and Y, and C,M,Y fixed values output circuit 32 for outputting fixed values of the three primary color component data C, M, and Y. The color conversion apparatus is also provided with four switches 34 to 40 for switching the three primary color component data C, M, and Y. The term `CIE` in the above description represents Commission Internationale de l'Eclairage.
The following describes the process of converting color component data in the LUT element. In a normal operation, the switches 34 and 38 are in contact with the side `a `, and the LUT element is thus separated from the LUT generating element. The input one-dimensional LUT 20 receives the color component data C, M, Y, and K to be fed to a target device, and cancels .gamma. correction made on every color component data C, M, Y, and K as discussed previously. The color component data C, M, and Y are input into the three-dimensional LUT 22 via the switch 34, whereas the color component data K skip the three-dimensional LUT 22 and are input into the output one-dimensional LUT 24.
The three-dimensional LUT 22 carries out color conversion of the input color component data C, M, and Y and outputs the color-converted data. For distinction, the data of three primary colors C, M, and Y prior to being input into the three-dimensional LUT for color conversion are expressed as C, M, and Y, whereas the data of three primary colors C, M, and Y output from the three-dimensional LUT after the color conversion are expressed as C', M', and Y'.
The three-dimensional LUT 22 stores combinations of the values of color-converted color component data C', M', and Y' corresponding to the combinations of the values of input color component data C, M, and Y. In accordance with a concrete procedure, the three-dimensional LUT 22 receives a combination of the values of input color component data C, M, and Y as an address specifying signal and reads out the corresponding combination of the values of color-converted color component data C', M', and Y'.
In case that the contents of the three-dimensional LUT 22 have been created in a manner discussed below, the color component data C, M, and Y are transformed from a color space proper to a target device to a standard color space not depending upon any device and further to a color space proper to a simulation device via the three-dimensional LUT 22. This enables the simulation device to obtain color component data that can be handled in the common color space with the target device.
In order to reduce the required storage capacity of the three-dimensional LUT 22, the number of combinations of the values of color-converted color component data C', M', and Y' to be stored therein is decreased. The decrease in number of combinations to be stored, however, causes deterioration of the quality of the printing results. In order to prevent deterioration of the quality with the decrease in number of combinations, interpolating arithmetic operations are carried out in the three-dimensional LUT 22 for the combinations not stored therein. The details of the interpolating operations are, for example, disclosed in U.S. Pat. No. 4,275,413.
The color-converted color component data C', M', and Y' output from the three-dimensional LUT 22 are input into the output one-dimensional LUT 24 via the switch 38. The one-dimensional LUT 24 also receives the color component data K output from the input one-dimensional LUT 20, makes .gamma. correction on every input color component data C', M', Y', and K, and outputs the corrected color component data C', M', Y', and K to a simulation device.
The following describes the process of creating the contents of the LUT for color conversion by the LUT generating element. The required measurement data are collected for both the target device and the simulation device, prior to the generation of the LUT for color conversion. Pre-defined, various values of color component data C, M, Y, and K are input into the respective devices for printing, and the results of printing obtained are measured with a spectral colorimeter to yield L*a*b* data. A table of the CMY data and L*a*b* data is then created for each device.
The switch 34 is brought into contact with the side `b` and the switch 36 with the side `c`, and the color component data C, M, Y, and K, which have been input into the target device, are thereby input into the input one-dimensional LUT 20 shown in FIG. 24. Among the color component data C, M, Y. and K input into the input one-dimensional LUT 20, the three primary color component data C, M, and Y are given to the matrix arithmetic operation circuit 26 for transformation of CMY data to L*a*b* data via the processing circuit 42 for formation of a polynomial, and transformed therein to L*a*b* data by an arithmetic operation using a transformation matrix A defined as Equation (1) given below. The values of L*a*b* data obtained by the transformation are compared with the values of L*a*b* data written in the table that has previously been created for the target device. In order to make the values obtained by the transformation approach the values written in the table, the transformation matrix A used in the matrix arithmetic operation circuit 26 is calculated by the least squares approximation. ##EQU1##
The switch 38 is then brought into contact with the side `b` and the switch 40 with the side `c`, and the data of the same values as the L*a*b* data written in the table that has previously been created for the simulation device are thereby input into the matrix arithmetic operation circuit 30 for transformation of L*a*b* data to CMY data via the processing circuit 44 for formation of a polynomial. The matrix arithmetic operation circuit 30 transforms the input L*a*b* data to three primary color component data C, M, and Y by an arithmetic operation using a transformation matrix B defined as Equation (2) given below. In order to make the values of color component data C, M, and Y obtained by the transformation approach the values of color component data C, M, and Y written in the table, the transformation matrix B used in the matrix arithmetic operation circuit 30 is calculated by the least squares approximation. ##EQU2##
The processing circuit 42 or 44 forms a polynomial of the data which are to be input into the matrix arithmetic operation circuit 26 or 30. This procedure enhances the precision of approximation of the CMY to L*a*b* transformation or the L*a*b* to CMY transformation in the matrix arithmetic operation circuit 26 or 30.
After the calculation of the transformation matrices A and B used in the matrix arithmetic operation circuits 26 and 30, the switches 36 and 40 are respectively brought into contact with the side `d`, so that predetermined, various fixed values of three primary color component data C, M, and Y are output from the C,M,Y fixed values output circuit 32. The output data C, M, and Y are processed by the formation of a polynomial, the CMY to L*a*b* transformation, the gamut mapping, the formation of a polynomial, and the L*a*b* to CMY transformation in this order, and successively written into the three-dimensional LUT 22.
These steps create the contents of the three-dimensional LUT 22 functioning as the LUT for color conversion. Conversion of color component data C, M, and Y by the three-dimensional LUT 22 thus prepared is equivalent to implementation of the above processing for the color component data.
The contents of the input one-dimensional LUT 20 and the output one-dimensional LUT 24 are created based on the results of measurement, which measures the .gamma. characteristics of the target device and the simulation device with respect to the four colors C, M, Y, and K when the results of printing by both the devices are measured with a spectral calorimeter. The contents of the gamut mapping circuit 28 are created based on the results of measurement, which measures the results of printing representing, for example, a gradation scale, for each color.
In the color conversion apparatus described above, data to be input into the matrix arithmetic operation circuits 26 and 30 are formed to polynomials, in order to enhance the precision of least squares approximation, by which the transformation matrices A and B used in the matrix arithmetic operation circuits 26 and 30 are calculated. When the three primary color component data C, M, and Y output from the C,M,Y fixed values output circuit 32 are written into the three-dimensional LUT 22 via the matrix arithmetic operation circuits 26 and 30, the colors of intermediate lightness having the enhanced precision of approximation are hardly affected by the transformation errors in the matrix arithmetic operation circuits 26 and 30. The precision of approximation is, however, not significantly improved for the colors of high lightness or low lightness. Such high-light or low-light colors may thus be significantly affected by the transformation errors and converted to different hues. The eyes of the man are rather insensitive to the colors of low lightness, but sensitive to the colors of high lightness. The change to different hues for the high-light colors is thus noticeable.
The type of paper used in the simulation device is often different from that used in the target device. Even when the contents of the three-dimensional LUT 22 are created to attain the color matching including the color of paper, since the very light color, such as the color of paper, is easily affected by the transformation errors, it is difficult to reproduce the color of paper used in the target device accurately on the paper used in the simulation device.
The method applied to solve this problem rewrites the contents of the three-dimensional LUT 22 in order to reproduce the color of paper more accurately by taking into account the effects due to the transformation errors. The color component data C, M, and Y representing the color of paper given to the simulation device is registered at an address specified by the combination of input color component data C, M, and Y representing white (that is, the combination of (C,M,Y)=(0,0,0)) in the three-dimensional LUT 22. The values of color-converted color component data C', M', and Y' registered at the address are thus to be changed to the values that can reproduce the color of paper accurately.
As discussed previously, the conversion of color component data is carried out by referring to the contents of the LUT and executing the interpolating arithmetic operations in the three-dimensional LUT 22. Rewriting the contents of the three-dimensional LUT 22 may accordingly deteriorate the quality of printing results. In accordance with a typical procedure, only a specific combination of color component data C, M, and Y representing the color of paper is replaced by another combination, and printing is carried out with a variation in only shade or gradation of a certain hue. In case that a certain color existing in the colors of varied gradation happens to be identical with an original color prior to the rewriting, the certain color is undesirably converted to a new color after the rewriting. This causes a color of different variation to appear in smooth gradation, which results in a distinct color skip.
In order to prevent such troubles, the contents of the three-dimensional LUT 22 should be rewritten not only for one combination but for a relatively wide range of combinations. This rewriting process is fundamentally based on the trial and error and thereby requires much time and skill of color evaluation.